Related Art
A wide variety of solid inorganic bodies is prepared in the chemical process and related industries. The spherical or spheroidal shape is useful for such bodies because of its desirable properties, such as better packing, higher strength, less "fine" particles, and better flow characteristics, to name a few. These spherical or spheroidal bodies find application primarily as catalysts, or as supports for catalysts, or as adsorbents and the like.
A number of processes are known to form solid inorganic bodies. For instance, in U.S. Pat. No. 3,656,901, Kummerle discloses that silica-alumina and silica particles are gelled by adding drops of aqueous colloidal sodium silicate or colloidal silica alumina to a solvent, such as an alcohol, ether-alcohol or amine. However, this patent does not disclose a method of obtaining substantially spherical particles having a diameter in a specified range.
In U.S. Pat. No. 3,844,978, Hickson discloses a process wherein hydrothermal crystallization is conducted using an aqueous slurry of hydrous sols and salts. The slurry is subsequently dewatered and dried to give solids which are then ground to a desired size. This patent does not teach the formation of small substantially spherical or spheroidal agglomerates.
In U.S. patent application Ser. No. 524,197, filed Aug. 18, 1983, now U.S. Pat. No. 4,507,396, Hickson describes a method wherein micron-sized particles are dispersed in a non-aqueous medium and agglomerated by the addition of an aqueous phase in the presence of colloidal particles to give a plastic mass which is then extruded or otherwise formed into particulate bodies.
In U.S. Pat. No. 3,258,311, Burzynski et al disclose a process for the formation of uniformly small spherical beads from alkali metal-silicates. The method comprises the steps of (1) combining the particle-forming ingredients comprising: (a) water; (b) a compound of the general formula xR.sub.2 O.ySiO.sub.2 where R is an alkali metal and the x/y ratio is greater than 0.24 (an x/y ratio of R.sub.2 O/SiO.sub.2 which characterizes the water soluble sodium silicates, and also generally the other water soluble alkali silicates); (c) dilute strong aqueous acid; and (d) an emulsifying agent; and (2) agitating the resulting mixture. The beads are usually about 1 micron to 1.5 millimeters in diameter. This process is disadvantageous because when it is used to make beads larger than 1.5 mm, they are not uniform in size.
In U.S. Pat. No. 3,140,251, Plank et al disclose the formation of spheroids by dispersing an aluminosilicate in a hydrosol, which is obtained by reacting an alkali metal silicate with an acid or an alkaline coagulant. The hydrosol may be dispersed through a nozzle into a bath of oil or other water-immiscible suspending medium to obtain spheroidally shaped bead particles of catalyst. However, the uniformly small size of the agglomerate cannot be controlled, and with high agitation the colloidal solution would form an emulsion.
In U.S. Pat. No. 3,296,151, Heinze et al disclose a process in which solid zeolite particles are wetted with water, mixed with a binder and kneaded into a paste which is extruded or otherwise shaped and dried. Heinze et al also disclose other agglomeration processes, including a process in which an aqueous sol is dripped into a water-immiscible liquid where the sol gels as it falls through a column of liquid. In both cases, spherical zeolite molecular sieves are produced, which have an undisclosed diameter or range of sizes.
In U.S. Pat. No. 3,515,684, McEvoy discloses the formation of fluidizable cracking catalyst particles. A dispersion of finely divided plastic particles of kaolin in water are intensely agitated in an oil to agglomerate the particles to provide a size distribution of the order of 15 to 150 microns in diameter suitable for catalysts for fluidized cracking. A disadvantage of this process appears to be that it is limited to producing particles having a diameter of 15 to 150 microns.
In U.S. Pat. No. 4,013,587, Fischer et al disclose a process for preparing alumina-containing particles which comprise the steps of: (a) mixing an aluminum hydroxide hydrosol with a high molecular weight natural organic material to form a mixture; (b) introducing the mixture in dispersed form into a water-immiscible liquid to form gel particles at elevated temperatures; (c) aging the particles in the liquid and then in aqueous ammonia; (d) recovering the particles; and (e) calcining the particles. The disadvantages of this process include the use of elevated temperatures, and use of aqueous ammonia which can be hazardous.
In U.S. Pat. No. 2,474,911, Pierce et al teach the preparation of micro-spherical gel particles in a continuous manner. A sol is introduced into a water-immiscible liquid such as an oil containing an emulsifier. The zone of turbulence is only at the bottom of a mixing column and the flow rate of the oil in the column is maintained to achieve continuous flow of the gel droplets. Pierce et al do not teach the obtaining of spherical particles having a diameter of 1-5 mm.
A few additional U.S. patents are of interest. In U.S. Pat. No. 2,384,946, Marisic discloses the formation of generally spherical hydrogel pellets. The pellets are obtained by spraying the hydrogel through an orifice into a gaseous or liquid medium.
U.S. Pat. No. 2,900,349, Schwartz discloses the preparation of inorganic oxide gels which have high resistance to attrition. In one embodiment, Schwartz describes the preparation of hydrogel spheroids by allowing the gel to fall or rise slowly through a column of hydrocarbon solvent.
In U.S. Pat. No. 3,004,929, Lucas et al disclose a process for the preparation and extrusion of silica-alumina catalyst supports. The catalyst supports obtained are usually pellets which are not uniformly spherical or in the size range described in the present invention.
In Australian Patent No. 127,250, Kimerlin et al disclose the production of finely divided gel particles which may be employed for catalytic adsorption and other purposes. The size is reported to be controlled by preparing inorganic gels in minute particles by emulsifying a hydrosol as the internal phase of a water-immiscible liquid, agitating the emulsion to prevent separation of the phases until the hydrosol is set and separating the gel particles. The disadvantage of this process is that the particles are of the order of 60-100 microns and thus are much smaller than the particles described herein.
Tauster, in the Journal of Catalysis, Vol. 18, No. 3, pp. 358-3680 (1980), discloses a process for impregnating the pores of particles with a metal salt by suspending them in a water-immiscible liquid, such as a hydrocarbon, and titrating the liquid with an aqueous solution of metal salt.
C. E. Capes, in "Agglomeration in Liquid Media" in the text Particle Size Enlargement, published by Elsevier Scientific Publishing Company, Amsterdam, The Netherlands, 1980, reviews a variety of applications of water-immiscible media in particle-forming processes. However, none of the processes disclosed by Capes, describes the steps or spheroidal product having a uniform diameter of about 1 to 5 mm as described herein.
In the Canadian Journal of Chemical Engineering, Vol. 47, pp. 166-170 (1969), A. F. Sirianni et al discuss a number of processes whereby finely divided solids in liquid suspension are agglomerated. The solids obtained may be separated as spherical bodies without regard to a substantially uniform size.
Additional sources of background information on agglomeration include "Agglomeration: Growing Larger in Applications and Technology" by J. E. Browning in Chemical Engineering, pp. 147-170 (Dec. 4, 1967); H. M. Smith and I. E. Puddington, Can. J. Chem. Eng., Vol. 38, 1916 (1960); J. R. Farrand, Can. J. Chem. Eng., Vol. 39, 94 (1961); and J. P. Sutherland, Can. J. Chem. Eng., Vol. 40, 268 (1962).
All of the art processes described hereinabove are not without some shortcomings. For one, it is often difficult to vary the composition and size of the solid body. Further, the products of these processes are often fine powders or chips having mechanical properties which may be unacceptable under the conditions of use. The present invention provides a method for forming small substantially spherical or spheroidal solid inorganic bodies which are particularly useful as catalysts and catalyst supports. The spherical or spheroidal shape provides additional strength, reduces breakage, improves packing, and the like. These spheroidal-shaped agglomerates have an average uniform diameter of greater than 0.5 mm, preferably between about 1 to 5 mm, and most preferably from 2 to 5 mm.
The present invention is an improvement over the agglomeration method claimed in U.S. Application Ser. No. 691,645 filed Jan. 15, 1985, now U.S. Pat. No. 4,599,321, with respect to making agglomerates which contain alumina. The present agglomeration method is carried out using alumina alone or alumina in combination with other inorganic materials, and in the presence of an aqueous solution which contains an acidic material. The agglomerates prepared by the present method have an increased crush strength and depending on the strength of the acidic material used, the agglomerates may have an increased surface area to volume ratio over agglomerates prepared in the absence of acidic material.